This invention relates to improved butterfly valves, having improved anticorrosive and pressure-resisting performances.
In order to improve the anticorrosive and pressure-resisting performances of the butterfly valve, it has been already proposed to provide a protecting layer on the fluid-contacting surfaces of the valve member as well as the valve body, the said layer being composed of fluoro-resin. In addition, there is provided a back-up intermediate elastic material layer between the protecting layer and the inside surface of the valve body per se, so as to improve the tightly contacting performance between the valve member and the valve body, thereby attaining an improveness in the pressure-resisting performance of the valve assembly. The both layers are united in practice into one piece which is attached reseparably to the inside surface of the valve body. This piece may be called "valve seat member" (refer to Japanese Patent Publication Sho-45-13307).
There is a further proposal to prevent fluid leakage from around the valve stem by provision of one piece of cup seal ring or of a stack of sealing rings so as to effectively seal off the relatively sliding surfaces of the stem and the valve body (refer to, for instance, Japanese Patent Publication Sho-46-15585).
As the fluoro-resin used for the lining material in the conventional butterfly valves, polytetrafluoride ethylene, hereinafter to be described briefly as PTFE, is predominantly utilized. On the other hand, as the material of the elastomer composing the said intermediate back-up layer, natural rubber or synthetic rubber is predominantly utilized. The term "intermediate" as used herein throughout the specification and the appended claims denotes as such, when the valve body per se and the valve seat member have been assembled together to constitute the valve body.
While the material: PTFE represents superior anticorrosive performance, it shows a rather high rate of gas permeability. Therefore, when such butterfly valves as covered by this substance are used in gaseous fluid pipe lines, such as those for conveying gaseous chlorine, the gas will so liably penetrate from inside through the protecting layer into its backup intermediate layer that the elastomer substance composing the latter may easily lose its elasticity, thereby the pressure-resisting and sealing function of the valve becoming soon defective.
With use of such conventionally and insidely covered butterfly valves, it has been further experienced that upon repeated on-off operation of the valve, the resilient back-up intermediate layer material will liably shift off from position on the inside surface of the valve body.
Another problem has been met with the inside covered butterfly valves such that on account of high pressure used in the assembly of the valve body elements, acting substantially on the mating flanges thereof, the elastomer material of the intermediate back-up layer of the valve seat ring, especially those existing in close proximity to these flanges will be subjected to a radially inwardly directing bulging. In extreme cases, these bulges will be transformed into inwardly convexed ridges which, when formed in reality, will destroy the tight fitness between the valve seat ring, especially the elastomer layer thereof, on the one hand, and the valve body per se, on the other. On the other hand, these bulge-out ridges will give rise to a locally concentrated severe wear on the inside valve body surface which is adapted for cooperating with the valve member. This drawback, when appearing, will substantially decrease the pressure-resisting performance of the valve assembly.
It has been further found that such deformation and/or positional shift of the elastomer layer will affect occasionally upon the sealing effectiveness at each end of both ends of the valve stem, thereby a fluid escapement through such defective sealed portions being disadvantageously encountered. According to our practical experience, such sealing loss as of the above kind can not be prevented completely even if the conventional cup type sealing ring(s) or vee seals be utilized. It has further been experienced that with use of these conventional cup type sealing rings at the ends of the valve stem, a functional defect will be rather rapidly encountered on account of severe wear appearing at the mutually rotating parts of the valve including the valve stem.